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Nov. 28, 2018 / 3:33 PM GMT / Updated Nov. 28, 2018 / 4:06 PM GMT

By Alexander Smith

Russia's weekend maritime clash with Ukraine illustrated the vast military imbalance between the neighbors.

No fewer than 11 Russian vessels surrounded Ukraine's two light military ships and one tugboat, ramming them before eventually opening fire on Sunday, Ukrainian President Petro Poroshenko told NBC News.

Poroshenko insists his country is under "extremely serious" threat of a land invasion, and that's why he had to declare martial law for 30 days in regions adjacent to Russia.

The gulf in military might was further driven home when Russia announced Wednesday it would deploy another of its S-400 surface-to-air missile systems to Crimea, the peninsula it annexed from Ukraine in 2014. The S-400 can track 300 targets simultaneously including aircraft, drones and other missiles mid-flight, and has a range of 250 miles. It will be the fourth S-400 in Crimea.

According to Poroshenko, Russia also tripled the number of tanks at a base near the Ukrainian border from September to October.

"We are now strengthening our defense on all the borders with Russia," Poroshenko told NBC News' Richard Engel on Tuesday.

The U.S. currently has around 200 soldiers stationed as advisers in Ukraine.

Ukraine says that Russia is attempting to take control of the Sea of Azov. The annexation of Crimea gave it control of the Azov’s only entrance, the Kerch Strait, choking access to Ukraine’s own ports.

In May, Russia opened a $3.69 billion bridge across the strait, and Ukraine says Sunday’s clashes were evidence the Kremlin is further tightening its grip.

Kiev is concerned about how to protect a 180-mile stretch between Crimea and the eastern part of Ukraine that's already run by Russian-backed separatists.

Most of Ukraine's navy was based in Crimea when it was annexed, meaning the military lost around 80 percent of its fleet to capture or defections. Russian President Vladimir Putin has offered to give the vessels back, although he described what is left as being a "wreck."

The three Ukrainian vessels detained by Russia in the Kerch Strait on Sunday.Russian Federal Security Service / Reuters

Since 2014, fighting in eastern Ukraine has claimed more than 10,000 lives. In 2015, the rebels briefly stormed the port city of Mariupol. Poroshenko says he believes they might try again.

"I have a document of intelligence in my hands," he said Monday, explaining his decision to impose martial law. He said the files contained a "detailed description of all the forces of the enemy located at a distance of literally several dozens of kilometers from our border, ready at any moment for an immediate invasion of Ukraine."

Some experts are skeptical.

"It is very unlikely that the rebels would attempt to mount a land assault and try to push the boundaries of the conflict further into Ukraine," says Emily Ferris, a research fellow at the Royal United Services Institute for Defence and Security Studies, a London think tank.